Optically active point defects in crystals have gained widespread attention as photonic systems that can find use in quantum information technologies. However challenges remain in the placing of individual defects at desired locations, an essential element of device fabrication. Here we report the controlled generation of single nitrogen-vacancy (NV) centres in diamond using laser writing. The use of aberration correction in the writing optics allows precise positioning of vacancies within the diamond crystal, and subsequent annealing produces single NV centres with up to 45% success probability, within about 200 nm of the desired position. Selected NV centres fabricated by this method display stable, coherent optical transitions at cryogenic temperatures, a pre-requisite for the creation of distributed quantum networks of solid-state qubits. The results illustrate the potential of laser writing as a new tool for defect engineering in quantum technologies.Comment: 21 pages including Supplementary informatio

The locking of the electron spin to the valley degree of freedom in transition metal dichalcogenide (TMD) monolayers has seen them emerge as a promising platform in valleytronics [1,2]. When embedded in optical microcavities the large oscillator strengths of excitonic transitions in TMDs allow the formation of polaritons which are part-light part-matter quasiparticles [3][4][5][6][7]. Here, we report that polaritons in MoSe2 show an efficient retention of the valley pseudospin contrasting them with excitons and trions in this material. We find that the degree of the valley pseudospin retention is dependent on the photon, exciton and trion fractions in the polariton states. This allows us to conclude that in the polaritonic regime, cavity-modified exciton relaxation inhibits loss of the valley pseudospin. The valley addressable exciton-trion-polaritons presented here offer robust valley polarised states with the potential for valleytronic devices based upon TMDs embedded in photonic structures and valley-dependent nonlinear polariton-polariton interactions.Single layers of transition metal dichalcogenides (TMDs) are two-dimensional (2D) direct band-gap semiconductors which exhibit pronounced exciton resonances with binding energies of around 0.5 eV [8][9][10]. The monolayer nature of TMDs gives rise to strongly confined excitons with Bohr radii of around 1 nm and large oscillator strengths evident from optical absorption as strong as 15% [8]. By embedding TMD monolayers in optical microcavities this huge oscillator strength has allowed the realisation of the strong light-matter coupling regime and the formation of part-light part-matter polariton eigenstates [3][4][5][6][7]. Polaritonic states inherit properties such as a strong nonlinear interaction and low effective mass from the constituent exciton and photon components. In other material systems, this has led to the experimental realisation of a wealth of rich nonlinear phenomena such as Bose-Einstein condensation [11], superfluid-like behavior [12] and optical spin switching [13]. The observation of exciton-polaritons in TMDs creates new opportunities in engineering the polariton-polariton interaction [14,15] in 2D materials. Moreover, TMD based polaritons are expected to inherit additional degrees of freedom of valley pseudospin and finite Berry curvature from their constituent excitons and trions which can be utilised in new valley-polaritonic implementations [16].In this article we report on the valley addressability of polaritons in MoSe 2 monolayers embedded in tunable microcavities. We report clear valley polarisation of both exciton-and trion-polaritons which show a strong dependence of the polarisation degree on the cavity detuning. In the bare flake fast exciton depolarisation occurs due to the Maialle-Silva-Sham (MSS) mechanism [17][18][19]. We demonstrate that in the strong coupling regime this depolarisation mechanism can be overcome and robust valleypolarised polariton states with much higher polarisation degrees can be achieved. We support this...

Two-dimensional transition metal dichalcogenides exhibit strong optical transitions with significant potential for optoelectronic devices. In particular they are suited for cavity quantum electrodynamics in which strong coupling leads to polariton formation as a root to realisation of inversionless lasing, polariton condensation and superfluidity. Demonstrations of such strongly correlated phenomena to date have often relied on cryogenic temperatures, high excitation densities and were frequently impaired by strong material disorder. At room-temperature, experiments approaching the strong coupling regime with transition metal dichalcogenides have been reported, but well resolved exciton-polaritons have yet to be achieved. Here we report a study of monolayer WS2 coupled to an open Fabry-Perot cavity at room-temperature, in which polariton eigenstates are unambiguously displayed. In-situ tunability of the cavity length results in a maximal Rabi splitting of ħΩRabi = 70 meV, exceeding the exciton linewidth. Our data are well described by a transfer matrix model appropriate for the large linewidth regime. This work provides a platform towards observing strongly correlated polariton phenomena in compact photonic devices for ambient temperature applications.

Cation-exchange has been used to synthesize PbS/CdS core/shell colloidal quantum dots from PbS starting cores. These were then incorporated as the active material in solar cell test devices using a solution-based, air-ambient, layerby-layer spin coating process. We show that core/shell colloidal quantum dots can replace their unshelled counterparts with a similar band gap as the active layer in a solar cell device, leading to an improvement in open circuit voltage from 0.42 to 0.66 V. This improvement is attributed to a reduction in recombination as a result of the passivating shell. However, this increase comes at the expense of short circuit current by creating a barrier for transport. To overcome this, we first optimize the shell thickness by varying the conditions for cation-exchange to form the thinnest shell layer possible that provides sufficient surface passivation. Next, ligand exchange with a combination of halide and bifunctional organic molecules is used in conjunction with the core/shell strategy. Power conversion efficiencies of 5.6 ± 0.4% have been achieved with a simple heterojunction device architecture.

We report the observation of a spin-flip process in a quantum dot whereby a dark exciton with total angular momentum L = 2 becomes a bright exciton with L = 1. The spin-flip process is revealed in the decay dynamics following nongeminate excitation. We are able to control the spin-flip rate by more than an order of magnitude simply with a dc voltage. The spin-flip mechanism involves a spin exchange with the Fermi sea in the back contact of our device and corresponds to the high temperature Kondo regime. We use the Anderson Hamiltonian to calculate a spin-flip rate, and we find excellent agreement with the experimental results.